1 Field of the Invention
The present invention relates to a novel polishing composition based on cerium, a process for its preparation and the use thereof in the polishing of glass, ceramics and other vitreous type materials. More especially, the present invention relates to a novel polishing composition based on the rare earths.
2 Description of the Prior Art
Among the polishing compositions used at the present time in the glass industry, those based on the rare earths and in particular on cerium are generally the most efficient. Different processes are known for the preparation of such compositions. Thus, it has been proposed in published German Application No. 2 426 110 to precipitate by means of sodium hydroxide, an aqueous solution of rare earth sulfates in the presence of molochite, and to filter, wash, dry and calcine the resulting product to obtain the desired polishing composition. Such a process does not enable production of a polishing composition which is completely satisfactory from an efficiency standpoint, primarily because of the heterogeneity thereof, its undefined structure and the lack of reproducibility of the final product. These disadvantages are specifically the result of the operating conditions of the process, according to which the concentration of the reagents varies during reaction and does not permit the obtention of defined products in a reproducible fashion; same also arise because of the existence of foreign compounds (molochite) and the presence of sulfates, the precipitation of which by sodium hydroxide yields mixtures of complex products, such as double sulfates, hydroxysulfates and hydroxides, the amounts and nature of which may vary throughout the course of the reaction.
It has further been proposed [Chem. Abstracts, 80, 51688 (1974)] to prepare cerium based polishing compounds by precipitating ceric hydroxide with ammonia from a solution of rare earth nitrate wherein the cerium had previously been oxidized. Such a process requires an additional stage of purification by recrystallization by the ceriammonium nitrate and the taking up of the calcined oxide in a dilute acid; the products obtained do not display satisfactory polishing efficiency. Furthermore, such compositions do not have satisfactory properties of homogeneity, structure and reproducibility.
A process is also known from U.S. Pat. No. 3,768,989 for the preparation of polishing compositions by forming a rare earth carbonate-wollastonite precipitate, separating the precipitate and calcining same. And from U.S. Pat. No. 3,262,766 it is also known to prepare certain polishing compositions by treatin commercial rare earth carbonate solutions with fluosilicic or hydrofluosilicic acid. The polishing compositions prepared by the processes described in the two aforenoted U.S. patents have disadvantages similar to those described in connection with the German No. 2,426,110.
To obviate the disadvantages of the aforementioned processes, in French Pat. No. 2,472,601, assigned to the assignee hereof, a novel process has been proposed for the preparation of polishing compositions, characterized in that it comprises the following stages:
(a) a solution of a cerium salt, a basic solution and a solution of at least one acid and/or salt, the anions of which are capable of forming insoluble compounds of rare earths, are mixed together simultaneously and continuously, with the number of equivalents of base being greater than or equal to the number of equivalents of cerium and the pH of the reaction medium being greater than 6;
(b) the resulting precipitate is filtered;
(c) the precipitate is dried; and
(d) the precipitate is calcined.
The process provides compositions having good homogeneity, reproducibility, suspension stability in water and good polishing efficiency. However, the aforedescribed process is not entirely satisfactory, because the anion customarily employed was fluoride and the problems usually associated with fluorides, in particular pollution by effluents, whether liquid or gaseous, are encountered.
The problem thus remains to develop a novel polishing composition which would satisfy the following requirements:
(i) to be effective, i.e., to permit the polishing of a glass surface generally clear-polished as rapidly as possible;
(ii) to be clean, i.e., not to streak the glass during the polishing process;
(iii) not to cause detrimental reactions on the surface, i.e., not to be too chemically active with respect to the glass (the problems of burning, orange peel finish, blisters, etc.);
(iv) to have an extended useful life, to be usable for the longest possible period of time without difficulty;
(v) to be stable in suspension, permitting the homogeneous distribution of the powder in the bath, without settling (the dispersion should remain effective upon placing the composition in suspension and throughout its entire working period of useful life);
(vi) absence of foam, as foaming causes overflowing, clogs the lines, reduces the yield and interferes with personnel management;
(vii) to be easy to again place into suspension after an extended work stoppage and decantation, even if the bath is charged with elements of polished glass (the phenomenon of "caking");
(viii) to be non-toxic, to avoid problems of skin afflictions and other diseases;
(ix) to be a regular product, of pleasing color and easy to use;
(x) not to adhere to the glass after polishing, thus permitting rapid cleansing; and
(xi) to be readily flocculated for the elimination of manufacturing waste.